The kinetics and erythropoietin requirements of morphologically unrecognizable erythroid progenitor cells (BFU-e, CFU-e) will be studied by in vitro culture assays after severe suppression of committed progenitors by BCNU injection and after severe suppression of pluripotential stem cells (CFU-s) and of erythroid progenitors by Myleran. Numerical and kinetic aspects of erythroid progenitors and of recognizable erythroblasts will be examined during accelerated erythropoiesis induced in mice by prolonged hypoxia.